Wholly aromatic polyester resins have long been known. For instance, 4-hydroxybenzoic acid homopolymer and copolymers have been described in the past and are commercially available. Such polymers commonly are crystalline in nature, have relatively high melting points or possess a decomposition temperature which is below the melting point, and when molten frequently exhibit orientation in the melt.
The homopolymer of p-hydroxybenzoic acid is a very high melting, insoluble material and, hence, very difficult to fabricate. Melting points as high as 610.degree. C. were quoted--see W. J. Jackson, The British Polymer Journal, December 1980, p. 155. In order to depress the high melting point of the homopolymer so as to make it melt fabricable, a variety of materials incorporating different types of comonomers were prepared over the years.
One such material is, for example, the resin made from p-hydroxybenzoic acid, isophthalic and/or terephthalic acids and 4,4'-biphenol as described in Cottis et al., U.S. Pat. Nos. 3,637,595 and 3,975,487. The polymer has outstanding high temperature properties; it can be molded to give articles of high modulus and strength. It is offered commercially by Amoco Performance Products, Inc. under the trade name of Xydar.RTM..
The main drawback of the prior art p-hydroxybenzoic acid copolymers is the relatively high cost associated with the use of an expensive comonomer, such as 4,4'-biphenol, substituted hydroquinones (e.g., phenylhydroquinone), naphthalene diols, naphthalene dicarboxylic acids, and hydroxy-naphthoic acids. Efforts to replace these expensive monomers with the significantly less expensive hydroquinone, which is disclosed as an equivalent of biphenol in the aforementioned U.S. Pat. Nos. 3,637,595 and 3,975,487, were made by several research groups. To date, none of these investigations were successful.
Study of the prior art shows that replacement of 4,4'-biphenol with hydroquinone leads to materials with inferior properties. The problem created by the introduction of hydroquinone is basically the following: at high terephthalate contents, high melting generally intractable polymers are obtained; tractability may be achieved at higher isophthalate levels, but the polyesters are relatively low melting and often display low second order glass transition temperatures which lead to low moduli and low heat distortion temperatures. For example, polyesters from p-hydroxybenzoic acid (PHBA) isophthalic acid (IA) and hydroquinone (HQ) were prepared by Deex, U.S. Pat. No. 4,377,681. At mole ratios PHBA/IA/HQ of 33.3/33.3/33.3 the material had a glass transition temperature of 110.degree. C.; when the above coreactants were used at ratios of 50/25/25, a Tg of 115.degree. C. was obtained.
The high melting points of a series of p-hydroxybenzoic acid/terephthalic acid/hydroquinone copolymers are graphically illustrated in FIG. 2 of the paper by G. W. Calundann, titled "Industrial Development of Thermotropic Polyesters", published in "High Performance Polymers: Their Origin and Development", R. B. Seymour and G. S. Kirshenbaum, Editors; pp. 235-249; copyright 1986 by Elsevier Science Publishing Co., Inc. The publication shows clearly (in FIG. 2) that hydroquinone polymers melt at considerably higher temperatures than their 4,4'-biphenol counterparts. The T.sub.m of the lowest melting composition is about 420.degree. C. FIG. 4 of the same publication (p. 243) indicates how one research group was able to depress the melting points of the subject resins by incorporating naphthalene diols, naphthalene dicarboxylic acids, and hydroxy naphthoic acids into them. From a purely technical point of view, the latter approach was a success; however, the modified polymers were still expensive due to the high cost of the naphthalene-based monomers.
The intractability of the hydroquinone-derived materials is discussed in Jackson, et al., U.S. Pat. No. 4,242,496. Column 2, lines 18-26 states:
"U.S. Pat. No. 3,637,595 discloses that aromatic liquid crystal polyesters prepared from terephthalic acid, hydroquinone and varying amounts of p-hydroxybenzoic acid melt in the general range of 800.degree. to 900.degree. F. Obviously, the melting point of these polymers is far too high and the thermal stability is insufficient to permit these polymers to be used in conventional melt-processing equipment."
It is further stated (column 2, lines 33-40) that a solution to the above problem
"was to incorporate a substituent on some of the aromatic rings of the polyester, preferably on the diol ring. For example, it is well known that use of chloro, methyl or ethyl hydroquinone lowers the melting point of the polyester. Although this approach can be used to lower the melting point, typically the mechanical properties are also substantially reduced".
The patent goes on to propose the use of phenyl hydroquinone (an expensive comonomer) as the best way whereby the melting point can be reduced to obtain tractable resins, without adversely affecting the mechanical properties.
As indicated earlier, polyesters forming oriented melts were made from a variety of substituted hydroquinones. See, for example, Lee et al., U.S. Pat. No. 4,600,765; Hutchings et al., U.S. Pat. Nos. 4,614,790 and 4,614,791; and Funakoshi et al., U.S. Pat. No. 4,447,593.
Readily, processible polyesters made from p-hydroxybenzoic acid, isophthalic and optionally terephthalic acid, hydroquinone and 3,4'- and/or 4,4'-biphenol, 3,4'- and/or 4,4'-dihydroxy diphenyl ether, 3,4'- and/or 4,4'-dihydroxy diphenyl sulfide are the subject of Dicke et al., U.S. Pat. No. 4,603,190. It should be recognized that once again an expensive monomer is necessary to obtain tractable melts. Similar situations are encountered in a host of other U.S. and foreign patents. See, for example:
Portugall et al., European Patent Appln. No. EP-257,558; PA0 Hisgen et al., European Patent Appln. No. EP-257,598; PA0 Hisgen et al., German Patent Appln. No. DE-3,629,208; PA0 Hisgen et al., German Patent Appln. No. DE-3,629,210; and PA0 Okamoto et al., World Patent Application No. WO-88/00,955.
As pointed out above, tractable materials result at high isophthalic acid levels but the products typically display undesirably low glass transition temperatures. Deex, U.S. Pat. No. 4,377,681 states (Column 1, lines 31-38):
"For example, liquid crystal copolyesters have been prepared from the following fairly rigid molecular species: p-hydroxybenzoic acid, hydroquinone and isophthalic acid. However, when ratios of the monomers are selected to provide tractable polymers, the glass transition temperature is generally low and the high temperature mechanical properties are reduced".
Attempts to increase the Tg of these products have been made. Thus, Deex, U.S. Pat. No. 4,377,681 claims copolyesters prepared from p-hydroxybenzoic acid, isophthalic acid, hydroquinone and 2,2-bis(4-hydroxyphenyl)propane. The preferred compositions contain from about 20 to about 35 mole percent of p-hydroxybenzoic acid units, and from about 5 to about 12 mole percent of 2,2-bis(4-hydroxy phenyl)propane (bisphenol-A) based on the total diphenol components. Glass transition temperatures of about of 175.degree. to about 190.degree. C. were observed. These values represent an improvement when compared to the Tg's of the polyesters which do not contain the 2,2-bis(4-hydroxyphenyl) propane (.apprxeq.110.degree. to 115.degree. C.). However, they must be considered low as they lead to heat distortion temperatures (HDT's) which are, at best, of the order of about 120.degree. to 140.degree. C.; moreover, the introduction of bisphenol-A lowers the degree of crystallinity as well as the rate of crystallization; in addition, mold shrinkage of these copolymers is unsatisfactorily high.
The dilemma facing those who have attempted the development of tractable, high HDT, hydroquinone/benzene dicarboxylic acid/p-hydroxybenzoic acid copolyesters is perhaps best illustrated by Example 1 of Cottis et al., U.S. Pat. No. 3,975,487. A polyester having excellent properties, based upon a 1:3 molar ratio of isophthalic:terephthalic acids, p-hydroxybenzoic acid, and 4,4'-biphenol was prepared. When this example was repeated using hydroquinone in place of biphenol and all isophthalic acid as the dicarboxylic acid, a polymer having poorer properties (i.e., a lower tensile strength and a lower modulus) was obtained (column 10, lines 60-63).
Copolyesters based on p-hydroxybenzoic acid (PHBA), hydroquinone (HQ), isophthalic (IA) and terephthalic (TA) acids are disclosed in Cottis et al., U.S. Pat. No. 3,637,595. The subject patent shows one example in which a resin incorporating all of the four monomers is described (example 10). The polymer was formed from 1.0 mole of PHBA, 0.5 moles of IA, 0.5 moles of TA, and 1.0 moles of HQ. It was poorly characterized; weight loss in air, at 752.degree. F. was apparently high, indicating thermal stability problems. Our own characterization of the resin produced by the preferred process utilized in this invention (see "Experimental", Comparative Example 1) showed that it possessed a low melting point (325.degree. C.); a low crystallization temperature (299.degree. C.); and a low Tg (110.degree. C.). Present indications are that if the polymer of Cottis et al. Example 10 had been produced by the process described in Example 10, it would have had even poorer properties than the polymer of Comparative Example 1 due to residual reaction solvent in the polymer functioning as a plasticizer, reducing the crystallization temperature further.
Thus, the elusive goal of developing a low cost hydroquinone-based crystalline polymer which (1) is meltprocessible below the decomposition temperature of about 415.degree. C., (2) has a melting point in the range of 340.degree.to 400.degree. C.; a crystallization temperature of 300.degree. to 340.degree. C.; and a crystallization rate of at least 1,000 counts per minute; and which (3) when filled with 30 percent by weight of glass fibers has an HDT of at least 240.degree. C., has not been achieved in the prior art. In fact, based on the prior art, it appears unlikely that polymers having these properties can be produced, particularly polymers consisting essentially of units (I), (II), (III), and (IV).
It is the general object of the present invention to provide novel, inexpensive, melt-processible hydroquinone poly(iso-terephthalates) containing residues of p-hydroxybenzoic acid, which form a highly tractable oriented melt phase; and which are capable of melt extrusion to form quality high performance fibers, films, three-dimensional molded articles, etc.
It is a further object of the present invention to provide novel, melt-processible hydroquinone poly(isoterephthalates) containing residues of p-hydroxybenzoic acid, which form a melt phase below 400.degree. C. in the substantial absence of polymer degradation, unlike many other polymers which include relatively high concentrations of the 4-oxybenzoyl moiety.
Finally, it is an object of the present invention to provide novel, melt-processible hydroquinone poly(isoterephthalates) containing residues of p-hydroxybenzoic acid, which are capable of forming products exhibiting high modulus and strength in combination with other desirable properties such as retention of properties at elevated temperatures and high HDT's (from about 240.degree. to about 280.degree. C.).
These and other objects, as well as the scope, nature and utilization of the invention will be apparent to those skilled in the art from the following detailed description.